Typically, photovoltaic devices have been based upon a semiconductor layer having both an ohmic contact to a conducting sheet on one side, and a rectifying contact, such as a p-n junction or a Schottky barrier, on the other side to effect charge separation. Many semiconductor materials have been tested for use in photovoltaic devices, with the most widely used material for solar cells being silicon.
In order to achieve reasonably high power efficiencies for solar cells, however, it has heretofore been necessary to employ single crystal silicon of exceptionally high purity. Costs for producing this exceptionally high purity single crystal silicon have been inordinately high and have prevented silicon solar cells from being cost competitive with more conventional means of producing electrical power in most applications. Thus, silicon solar cells have only been used to any significant extent in applications where cost is not a controlling factor, such as on space vehicles or in power sources employed in remote areas.
More recent work has been directed towards producing efficient solar cells from amorphous silicon-based materials in order to lower the costs of such cells. One particular cell of this type has been produced from an alloy of amorphous silicon and hydrogen deposited from a glow discharge in silane. A p-i-n structure for this type of solar cell is described by Carlson and Wronski in which a few hundred angstroms of boron-doped amorphous silicon were deposited on an indium-doped tin oxide substrate followed by a layer of "intrinsic" or undoped amorphous silicon having a thickness of about 1 .mu.m. Thereafter, several hundred angstroms of phosphorus-doped amorphous silicon were deposited and an aluminum electrode was evaporated onto the top of the p-i-n structure to form a low-resistance contact to the n layer. See Carlson, D. E. and Wronski, C. R., Appl. Phys. Letts., 28 (11), 1976, pp 671-673. The maximum power efficiency obtained for this cell, however, was only about 2.4% under air mass 1 (AM 1) illumination.
Later developments of solar cells produced from amorphous silicon-based materials deposited from glow discharge resulted in a Schottky barrier structure having power efficiencies of up to about 5.5%. See Wronski, C. R., Trans. Electron Devices, Ed-24(4) 1977, pp 351-57. Schottky barriers were produced by vacuum evaporation of semitransparent metal films on the amorphous silicon layers. In the study of this Schottky barrier cell, it was concluded that the diffusion length of the minority carriers (L.sub.m), in this case holes, was only about a few tenths of a micrometer and that hole transport was the limiting factor in cell efficiencies. The short diffusion length for the minority carriers was a particular problem because the absorption thickness for solar radiation in amorphous silicon was found to be in the order of about 1 .mu.m. This means that carriers generated at distances of more than a few tenths of a micrometer from the barrier had little chance of reaching the barrier where they could contribute to the photocurrent generated. Thus, the author concluded that the efficiency of such amorphous silicon solar cells could be increased by improving the diffusion length of the minority carriers. Other authors have agreed with this conclusion. See, for example, Carlson, D. E., Transactions on Electron Devices, Ed 24 (No. 4) 1977, pp 449-453.